Many modern communication systems use linear modulation techniques for encoding and decoding information. These techniques are typically amplitude and/or phase modulation techniques that map a symbol alphabet to a set of signals. For example, one of the simplest modulation techniques is pulse amplitude modulation (PAM), which maps a symbol alphabet {a0, a1, . . . , aN-1} to N transmission levels. Other examples of linear modulation techniques include phase shift keying (PSK) that maps the symbol alphabet to N phases, and quadrature amplitude modulation (QAM) that maps the symbol alphabet to both amplitude and phase shifted symbols.
Techniques for demodulating a linearly modulated signal to recover the original input, such as de-convolution and de-correlation, are well known and fairly straightforward to implement. It is possible for an unauthorized third-party to gain access to the transmission channel, intercept the signal, use standard demodulation techniques to demodulate the signal and recover the input, thereby compromising the security of such a transmission system. It would be desirable to have a transmission system that would provide additional security at the physical layer, and prevent unauthorized parties from eavesdropping.